1. Technical Field
The present disclosure relates to servomechanisms, and in particular, to servomechanisms having jet pipes.
2. Discussion of the Art
Servomechanisms, such as servo-valves and rotary and linear servo-actuators, are well-known for use in applications requiring a high degree of position accuracy. Servos generally incorporate feedback systems. For example, closed-loop, negative feedback systems compare the control input to the actual position at the output of the device. Error signals relaying differences between the actual and desired values are used to drive the servo to reduce or eliminate the error and operate accurately. Servos can generate mechanical output through various means, including electric, electromagnetic, hydraulic and pneumatic input and combinations thereof, in which a first input (e.g., an electromagnetic force) can be force amplified using a second input (e.g., hydraulic pressure). Furthermore, servos can be direct drive devices, see e.g., U.S. Pat. No. 3,678,951, or multi-stage devices in which the input force of the preceding stage is amplified by a subsequent stage.
An example of the a multi-stage servo-actuator is disclosed in U.S. Pat. No. 3,221,760, which describes an electro-hydraulic servo having an electromagnetically driven flapper first stage and a pilot spool second stage. A mechanical force feedback device, such a feedback spring, links the first and second stages, specifically the spool to the flapper. Energizing the electromagnet causes the armature to pivot the flapper to close off one nozzle orifice more than the other, which creates a pressure differential across the spool so that it changes position. Movement of the spool causes a pressure difference across the outlet flow paths to drive the actuator device, here a piston-cylinder arrangement. Movement of the spool also imparts a returning force to the flapper through the feedback device.
While such servos can be of various constructions, flapper-type, jet pipe-type, and combinations thereof, are common. Generally, flapper-type servos use a flexible or movable flapper member to restrict flow through one of two nozzle orifices communicating with an associated output pressure port, such as disclosed in the aforementioned U.S. Pat. No. 3,221,760. Jet pipe-type servos feed the amplifying media through a nozzled tube or jet pipe that is movable to direct the media to one of two receiver orifices communicating with an associated output pressure port, see e.g. U.S. Pat. No. 3,678,951. Combinations of the two can have the jet pipe mounted to a flapper, see e.g., U.S. Pat. Nos. 3,584,649 and 3,621,880.
Jet pipe-type configurations are generally preferred over flapper-type configurations in contaminated environment applications, such as jet engine applications in the aircraft industry. Servos with jet pipes are generally less sensitive to contamination due to the permissibility of larger nozzle orifices and their better ability to maintain operation, even if sluggish, with a clogged orifice.
In addition to the issue of contamination, it is typically desired to isolate the electromagnetic coil(s) from the hydraulic fluid. Early jet-pipe servos were constructed so that the armature and the feed tube providing hydraulic fluid to the jet pipe were an integral unit or were an extension of one another such that there was no separation between the driven and driving members. The aforementioned U.S. Pat. Nos. 3,221,760 and 3,678,951 are exemplary of the construction of conventional dry coil servo valves, which use a thin-walled, flexible seal tube disposed between the housing and the flapper or jet pipe to isolate the coils from the fluid while permitting movement of the flapper or jet pipe without causing excessive (for some applications) coulomb friction hysteresis. The use of the flexible seal tube is more complex to manufacture, and in applications where higher hysteresis is acceptable it may be replaced by an elastomeric O-ring. However, O-rings are generally unacceptable for applications with large variations in fluid and ambient temperatures and/or where the fluid can cause excessive swelling of the elastomeric material. Yet, neither may be acceptable for applications where near-zero hysteresis is required, reliability is critical, and induced vibration can cause erratic performance of the servo.